Catheter delivery systems for trans-luminal delivery of implants, particularly self-expanding stents, have a rich history in the patent literature. Early proposals were for a simple sheath radially surrounding the radially-compressed stent at the distal end of the catheter system, the sheath being pulled back proximally, to release the stent from its bed, progressively, starting at its distal end of the bed, within the stenting site or stenosis of the bodily lumen in which the catheter delivery system had been advanced. Readers will appreciate that, because the stent is self-expanding, it is pressing on the luminal surface of the surrounding sheath, up to the moment of its release from the sheath. Thus, friction forces between the stent and the surrounding sheath must be taken into account when devising a delivery system that will allow the sheath to slide proximally over the full length of the outwardly-pushing, self-expanding stent.
The problems of friction will increase with the length of the stent, and the pressure on delivery system designers is to deliver ever-longer stents. Furthermore, there is steady pressure on stent delivery system designers to come up with systems that have ever-smaller passing diameters at the distal end of the catheter. The conventional unit of dimensions for diameters of systems to advance along a bodily lumen is the “French” which is one third of a millimeter. Thus, one millimeter is “3 French”. To be able to reduce the passing diameter of a delivery system, for example from 7 French to 6 French, is a notable achievement.
One way to respond to the challenge of friction forces between a proximally withdrawing sheath and a self-expanding stent confined within it is to adopt a “rolling membrane” sheath system, in which the sheath is at least double the length of the stent that it surrounds, being doubled back on itself at a point distally beyond the distal end of the stent. Then, proximal withdrawal of the radially outer doubled back portion of the sheath length will cause the “rolling edge” between the outer and inner sheath portions to retreat proximally, rolling proximally down the length of the stent, to release the stent progressively, as with a single layer surrounding sheath.
Regardless of whether a conventional or rolling membrane sheath system is employed at the distal end of a stent delivery system, the delivery system requires some form of deployment mechanism provided at the proximal end of the stent delivery system to enable an operator to control at the proximal end the deployment of the distally located stent inside a patient. Typically, the stent is provided on the distal end of a push rod that extends from the proximal end to the distal end of the system. With this push rod held stationary, the user operates such a mechanism at the proximal end, resulting in the sheath system being pulled back, thereby deploying the stent, as described above.
One stent deployment mechanism is disclosed in U.S. 2007/0244540 A1 (here “D1”), which is incorporated by reference in its entirety into this application. This mechanism involves the use of a thumb slider that is repeatedly translated distally and proximally, with each progressive proximal movement effecting progressive retraction of the sheath. A disadvantage of this deployment mechanism is the inability to deploy the stent in only one, or at least only a few, translations of the deployment mechanism. For lengthy stents, deploying the stent using this mechanism would prove a laborious task, requiring many translations. However, once the distal end of the implant is in place on the wall of the lumen in the body that is receiving the implant, a swift retraction of the sheath, to deploy the remaining length of the implant in one smooth stroke, is not available from this device.
D1 teaches the attractiveness of a hand unit that is physically small. The sheath of D1 is not a roll back membrane. Were it to be a roll back membrane, the distance it would have to be pulled back proximally would be doubled. The present invention aims to provide a simple and easy to manufacture hand unit that is small in size but yet is capable of deploying a lengthy implant covered by a roll back membrane.